Learn Portuguese (Portugal): Numa ação, o essencial é a consequência. - In an action, the essential thing is the consequence.

By placing the masculine singular article o before an adjective, we turn it into a noun. Here, o essencial translates to 'the essential thing' or 'that which is essential'. This is a very common way to talk about abstract concepts in Portuguese without having to literally use the word for 'thing'.

Could I just say a coisa essencial instead of o essencial?

You could, but using o plus the adjective is the most natural and idiomatic way to express this in Portuguese. It sounds much more elegant than translating 'the essential thing' word-for-word.

Why is it Numa instead of just Em uma?

In Portuguese, the preposition em (meaning in, on, or at) naturally contracts with the indefinite article uma (a or an) to form numa. It is highly recommended to use the contracted form in everyday speech and writing, as leaving them separated sounds very robotic.

How do we know ação is a feminine noun?